1. Field of the Invention
The present invention relates generally to semiconductor wafer marking devices and more specifically to a double pulse inker network having electronically adjustable parameters for use with a probing machine and a tester.
2. Description of the Prior Art
Integrated circuits are commonly manufactured on thin slices of semiconductor crystal material called wafers. Each wafer contains a large number of autonomous circuits which are later separated, as by scribing and fracturing, into individual chips or dice. Each die is then mounted into an individual package for sale.
Due to a variety of causes, some or all of the dice forming a particular wafer may be found to be defective. In fact, for many integrated circuits, only a small fraction of the total number of dice manufactured might perform satisfactorily. To avoid packaging defective dice, which may significantly contribute to the manufacturing costs, each individual die is usually tested before being divided from the wafer.
Although integrated circuit testing may be partially or fully automated, according to either procedure the wafer is first positioned so as to align a first die under a probe card which is designed to make electrical connections with particular points on the die. Next, the wafer is brought into contact with the probe card and electrical tests are performed by a tester on the individual die. Upon completion of the testing of the first die, the wafer is lowered and moved, as by stepping motors of a probing machine to position a second chip under the probe card. The testing procedure is then repeated on this second die, and so on. Defective dice are marked for later identification.
Numerous methods of marking have been employed such as scratching the die or destroying the die with a charged point or laser beam. Although effective in marking the die, these marking methods are destructive, and if it is later discovered that the testing was not performed satisfactorily, it is not possible to salvage the large number of formerly good dice which have been destroyed. Destructive testing is also incompatible with sorting of the dice by grade.
A more common method of dice marking is by the application of a spot of ink, such as with an inker. Typically, an inker includes a reservoir for the ink, a thin tube or quill which the ink and an ink-directing wick, such as fishline, pass through, and a solenoid and plunger for driving the fishline back and forth in the quill. The solenoid is used to drive the plunger and consequently the fishline down into contact with the die, permitting the ink to flow along the line onto the die. The longer the line is in contact with the die, the larger the dot size. If the line is driven down too far, it will skip along the surface of the die producing a large dot which may extend onto an adjacent die.
Heretofore, manufacturers of testers provided a fixed drive signal to drive their inker. The signal consisted of a pulse of from 5 to 30 milliseconds in width and having an amplitude of from 30 to 90 volts, the parameters having been set by the tester manufacturer. The adjustments provided are mechanical in nature, such as by spring loaded screws to limit the solenoid travel. Such adjustments are usually quite gross and tend to work loose due to vibration caused by the solenoid.
In order to produce a consistent dot size large enough to be easily recognized and small enough not to extend onto adjacent dice, compensation must be made for many variables. Not only are inkers which are produced by different manufacturers used on a given tester, but a large variety of ink types are also employed. Usually, the ink is supplied by the user (the manufacturers of testers seldom supply ink), which may be drafting ink, epoxy ink or some "home brew" such as food coloring and water with perhaps glycerin added for viscosity. The viscosity of the ink also varies because of changes in the ambient temperature.
Another major problem is due to the drying of the ink surface or, skinning, during operation. Should a large number of dice pass the test, and thus not be marked, by the time a defective die is encountered the ink may have skinned, preventing reliable marking. Since the first operation of the inker following skinning usually cleans the inker, subsequent defective dice will be properly marked, thus making detection of the skinning problem more difficult.